Acoustic echoes occur primarily in certain types of communication in which a remote user terminal comprises one or more directional microphones and one or more loudspeakers instead of an earpiece. Examples include audioconference equipment and hands-free telephones, such as mobile telephones. The source of the echoes is simple: failing special precautions, sound emitted by the loudspeaker(s) is reflected many times (from walls, the ceiling, etc.), constituting as many different echoes which are picked up by the microphone(s) on the same terms as wanted speech. Thus the combination of the loudspeaker(s), the microphone(s), and their physical environment constitutes an echo generator system.
The acoustic echo problem has been the subject of much research, both in the case of single-channel systems (one microphone and one loudspeaker) and in the case of multichannel systems (a plurality of microphones and a plurality of loudspeakers). The echo problem in the multichannel situation is similar to that in the single-channel situation except that all possible acoustic couplings between the various microphones and loudspeakers must be considered.
The echo processing techniques most widely used include echo suppression techniques using gain variation and echo cancellation techniques using adaptive filtering.
In a variable-gain echo suppression system, a receive gain is applied to the signal for application to the loudspeaker (the direct signal at the input of the echo generator system) and a send gain is applied to the signal coming from the microphone (at the output of the echo generator system), forming the return signal. An echo suppression system of this type is described in French Patent No. 2 748 184.
Receive voice activity detectors (RVAD), send voice activity detectors (SVAD), and double speech detectors (DSD) typically supply the necessary information to the modules that calculate the send and receive gains. Thus when the remote party is speaking (detected by the RVAD), the send gain is reduced to attenuate the echo. If the local party begins to speak (detected by the SVAD), this constraint on the send gain is removed and the receive gain is reduced. In the event of double speech (both parties speaking simultaneously, detected by the DSD), either a comparator determines which speaker is louder and gives priority to that speaker's sending direction or an intermediate setting of the send and receive gains is established.
In an acoustic echo canceller (AEC) using adaptive filtering, an identification filter estimates the acoustic coupling between the loudspeaker and the microphone and generates a signal that is used to cancel the echo. The identification filter is conventionally a programmable finite impulse response filter whose coefficients need to be adapted by a predetermined algorithm for updating coefficients using an adaptation step. The coefficients are adapted on the basis of the signal to be applied to the loudspeaker. An echo canceller of this type is described in French Patent No. 2 738 695.
A variable gain echo suppression system is often combined with an echo canceller to eliminate the residual echo that remains after echo cancellation.
However, the above-mentioned echo processing systems have the drawback that they are not able to take account of variations in the acoustic coupling between the loudspeaker and the microphone if those variations are independent of the signal applied to the loudspeaker.
This is the case, for example, if there is an external facility for adjusting the sound level reproduced by the loudspeaker (for example by means of a potentiometer). Any variation in the reproduced sound level modifies the acoustic coupling between the loudspeaker and the microphone and therefore the echo(es) picked up by the microphone. The echo processing system takes account only of the signal that is applied to the loudspeaker, and not of the sound that is actually reproduced by the loudspeaker, and is therefore unable to take this kind of modification of the acoustic coupling into account in its calculation process.
For example, if the sound reproduction level is reduced after the system has been initialized with a maximum sound level setting, in a double speech situation the remote speech emitted by the loudspeaker may be broken up or truncated.
Similarly, if the microphone and the loudspeaker in the communications terminal being used are physically independent of each other, the distance between them may be varied, which varies the acoustic coupling between the loudspeaker and the microphone, with the same consequences.
The problem is the same in a multichannel situation except that it generalized to the multiple couplings between the various microphones and loudspeakers.